1. Field of the Invention
The present invention generally relates to tracking the dependency of commands within a command queue.
2. Description of the Related Art
Computing systems often include central processing units (CPUs). Often requests to execute commands are made to the CPU from other devices within a system. Examples of devices which may make a command request to a CPU include a video card, sound card, or an I/O device within a system. Input or output (I/O) traffic for a CPU can be classified as Inbound or Outbound. Inbound traffic comes from an I/O device into the CPU. Outbound traffic is initiated from the CPU and goes to an I/O device. As commands come into the CPU they are placed into a command queue. The command queue is used to maintain order and priority amongst the different commands the CPU receives. In some CPUs the command queue is a FIFO (first in first out) queue, or one that executes the commands in the order in which they were received.
A command queue may include commands which are dependent on commands within the same or other command queues. An example of a dependent command is a command to read from an address received after a command to write to the same address. If the read command is executed before the write command the data read may not be current (not updated the data from the write command, i.e. “stale”). Therefore, it is essential that a command queue issue commands to be executed by the CPU in an order such that the adverse effects of command dependencies are not realized (e.g. data read is not stale).
One way to avoid the adverse effects of command dependencies is to enforce ordering on the I/O commands in software. In other words software may be required to be written to not allow load or store commands to the same memory location to be in-flight at the same time. However, this solution is not desirable because it limits the ability of software to use DMA (direct memory access) engines and pipelined access to memory.
Another way to avoid the adverse effects of command dependencies is to use a second FIFO command queue containing multiple pointers. The pointers target commands within the first command queue. When two commands within the first command queue are set to use the same address, a stall pointer is set to mark the new “colliding” command. The “colliding” command and all commands behind it are stalled until all commands ahead of it, including the command that targeted the same address, have exited the queue. The drawbacks to this solution are that it forces the use of a second FIFO, prevents commands that have no dependencies from going ahead of commands that have dependencies, and commands are over stalled because this approach only looks at one dependency at a time, not every dependency individually.
Therefore, there is a need for an improved method and apparatus for tracking command dependencies within command queues.